Introduce FSInputAccessor and use it

Backported from the lazy-trees branch. Note that this doesn't yet use
the access control features of FSInputAccessor.

src/libfetchers/fs-input-accessor.hh

@@ -0,0 +1,33 @@
+#pragma once
+
+#include "input-accessor.hh"
+
+namespace nix {
+
+class StorePath;
+class Store;
+
+struct FSInputAccessor : InputAccessor
+{
+    virtual void checkAllowed(const CanonPath & absPath) = 0;
+
+    virtual void allowPath(CanonPath path) = 0;
+
+    virtual bool hasAccessControl() = 0;
+};
+
+typedef std::function<RestrictedPathError(const CanonPath & path)> MakeNotAllowedError;
+
+ref<FSInputAccessor> makeFSInputAccessor(
+    const CanonPath & root,
+    std::optional<std::set<CanonPath>> && allowedPaths = {},
+    MakeNotAllowedError && makeNotAllowedError = {});
+
+ref<FSInputAccessor> makeStorePathAccessor(
+    ref<Store> store,
+    const StorePath & storePath,
+    MakeNotAllowedError && makeNotAllowedError = {});
+
+SourcePath getUnfilteredRootPath(CanonPath path);
+
+}